Efficiency improvement targeting Collection and Upload

[maxsibilla]

Collection and Upload index procedure is very different from other entity types, similar to each other though.

Collection.datasets and Upload.datasets are both generated by on_read_trigger. This can be time-consuming when a collection has lots datasets. For instance, 3ae4ddfc175d768af5526a010bfe95aa has 211 datasets, the GET request takes 8 seconds to generate a 3.6MB payload.

As an additional efficiency improvement,

• Rename Collection.dataset_uuids (used by POST only) to Collection.dataset_uuids_to_link (no side effects since no Collection creation being used by other services). Also update the trigger method to use this new field. (Karl? Since he made Collection creation/update using the generic POST/PUT)
• Rename Collection.datasets to Collection.dataset_uuids with only returning a list of uuids (requires updating the neo4j quey and corresponding search-api tweaks)
• Rename Upload.datasets to Upload.dataset_uuids with only returning a list of uuids (requires updating the neo4j quey and corresponding search-api tweaks)

HM related card: hubmapconsortium/entity-api#632

[maxsibilla]

Not applicable after indexing rework